Members of the nuclear receptor superfamily play critical roles in development and homeostasis by regulating gene expression in response to the binding of small molecular weight ligands. The central hypothesis of this proposal is that transcriptional responses to regulatory ligands are determined by the exchange of corepressor complexes for one or more coactivator complexes. Sequential or combinatorial recruitment of biochemically distinct coactivator complexes is proposed to underlie cell and gene-specific responses to a particular ligand. Aspects of this hypothesis will be tested using biochemical and cell-based assays and by determining the consequences of knocking out the NCoR and SMRT genes in mice. The biological focus of these studies will be to explore whether distinct coactivator and corepressor complexes underlie specific programs of hematopoietic differentiation that are controlled by retinoic acid receptors (RARs) and peroxisome proliferator activated receptors (PPARs). Three specific aims are proposed: The first is to test the hypothesis that transcriptional activation of RAR and PPARgamma target genes requires sequential or coordinate recruitment of p160/CBP and DRIP/TRAP/ARC coactivator complexes. These experiments will address the question of whether these complexes act sequentially or combinatorially, and whether both complexes are required on different PPARgamma and RARalpha target genes. The second specific aim is to determine the roles of NCoR and SMRT in the control of hematopoiesis. The NCoR knockout mice die around embryonic day 16 due to profound anemia. Experiments are proposed to define the molecular basis for this phenotype and determine the roles of NCoR and SMRT in regulating the differentiation of granulocytes and macrophages. The third specific aim is to test the hypothesis that ligand-dependent inhibition of NF-kB-target genes by PPARgamma other nuclear receptors involves the recruitment of inhibitory molecules to the CBP coactivator complex. PPARgamma, RARalpha and many other ligand-dependent nuclear receptors appear to exert important biological effects by inhibiting the activities of other signal-dependent transcription factors in a ligand-dependent manner. Preliminary studies suggest important roles for CBP/p300 and additional receptor-associated proteins in the transrepression process. Together, the proposed studies are intended to lead to new insights into the biochemical and biological roles of coactivators and corepressors in the regulation of gene expression by RARs and PPARs. This knowledge may suggest new approaches for the development of nuclear receptor ligands useful in the treatment of a broad spectrum of human diseases.